The Gold Trophy that for the Monoceros is now unlocked by any player finishing in the top 100 of the weekly individual tournament on iOS and Android. It is still possible to get a Gold Trophy as a mystery prize & it still costs 4500 Diamonds.

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Unlike the other Tier 5 tanks in Iron Force, the Monoceros isn't unlocked when you reach a certain rank, it's unlocked when you obtain a Gold Trophy. There are two ways to obtain a Gold Trophy: finish in the top 50 places in the weekly Individual Tournament or earn the Gold Trophy via the Mystery Prize in the Daily Bonus system. Because neither of these options are related to a player's rank (and the second option is completely random), it’s possible for players of any rank to have the Monoceros tank.